The invention relates to fluid control and actuation systems. More particularly, it pertains to the application and release of friction elements used in a power transmission for a motor vehicle.
In an automatic transmission the various speed ratios of the transmission result by selectively engaging and disengaging friction elements, such as hydraulically actuated clutches and brakes. The applied and released condition of the friction elements connects and disconnects elements of the planetary gearsets to produce the gear ratios. The friction elements are applied and released in response to the pressurized and vented state of the friction elements.
In such a transmission operating according to the principles of direct electronic shift control, the pressure of each friction element is controlled by a solenoid-operated control valve in response to a command signal produced by an electronic controller that repetitively executes computer coded control algorithms. The command signal automatically produces upshifts and downshifts in accordance with a control strategy that relies on input signals produced by sensors, which monitor operating conditions of the vehicle, its engine, and requests of the vehicle operator related to powertrain performance.
The magnitude of torque transmitted by the various friction elements in the several gear ratios is reflected in the magnitude of pressure applied to each friction element. When the magnitude of transmitted torque is high, the magnitude of actuating pressure is high. Generally, during operation in the lowest forward drive gears and reverse gear, the transmitted torque magnitude is high. In the higher forward drive gears, the magnitude of transmitted torque is low. For example, the control pressure supplied to a friction element in reverse gear may be approximately 300 psi., whereas the control pressure supplied to a friction element in the higher gears may be approximately 150 psi.
During operation in gear ratios where a high control pressure is needed, a large gain is required to control accurately the control pressure in response to the pressure command produced by the controller. A lower gain is required when control pressure is low. To produce accurate shift control, it is preferred that incremental changes in the command signal produce relatively small changes in control pressure.
Generally, a control system for an automatic transmission produces line pressure up to 300 psi. Control pressure is the result of regulating line pressure by controlling the size of the hydraulic connection between the line pressure source and the control element, and the opening between the control source and a relatively low exhaust pressure source, generally the sump or transmission reservoir where excess hydraulic fluid is held for return to the system.
For those friction elements that are employed in both high control pressure ratios and low control pressure ratios, it is preferred to alternate between a low gain and a high gain control system at an additional cost rather than to accept the compromised resultant shift quality